Raspberry Pi is a Linux machine. We all know that Linux is fully configurable to develop microcontroller programs and flash chips using open-source tools like AVRDude. With Raspberry Pi there is no problem to have GCC tools running where you can compile AVR programs and then later to flash using AVRDude. To program chips you would normally connected programmer adapter to one of USB ports that are accessed by AVRDude. But Kevin wanted to get rid of need for adapter and use RasPi hardware instead. Among exposed Raspberri Pi GPIO header pins there is a SPI interface that can be used to talk to AVR chips directly.
So he added new programmer “linuxspi” in AvrDude that accessed AVR chips using hardware SPI. Other functionality remains pretty same as you’d expect from AVRDude with majority chips supported. IT lack support of TPI programming as TPI seems not working directly from SPI, but future plans may include this functionality as well. Current avrdude with SPI programmer is available at Git.